We got a hot tub about 2 months ago. I researched heavily to see if it would be okay to use at a lower temp. It seems that you don't want to raise your body temp to over 101. The baby's temp is hotter than mom's and "they" say raising the mom's core temp to over 101 can cause damage to the fetus. Most hot tubs are set to 104. Even before pregnant, I tested mine out at 98 and it was just fine for me. I raised the temp to 102 and had DH sit in it for an hour and we took his temp once every 15 minutes. It never went above 98.9. Since I won't be going in it unless it's 99 or under, I think I'm fine. I worry about a bath I took last week to relieve cramps, but I'm reassured by how difficult it was to raise DH's temp for sitting in a hot tub for over an hour (<---yes I'm a nerd...he did this for me last night to reassure me about my bath...such a good guy).
Anyway, in my previous research, I also read an article which discussed an experiment done on women, core temp, and hot tubs. They have a bunch of women sit in a hot tub set at 104 degrees with thermometers in their vaginas (if I remember correctly) to measure core temp. All women voluntarily got out of the hot tub before their temps were over 101 because they felt uncomfortable. The take away I got from there was that it's likely you will get out of a tub because you felt hot and uncomfortable well before your core temp got so high it hurt the baby.
Hope that helps.